This invention relates, generally, to a prosthesis and more particularly to a bone prosthesis having an intramedullary fixation stem which is somewhat flexible and also comparable in flexibility to that of the surrounding supportive cortical bone. This structure gives great advantages and overcomes the difficulties encountered with prior art devices as will be more fully described and explained herein below.
Heretofore, prosthesis components and particularly femoral prosthesis components which are utilized for surgical reconstruction of a human hip joint have incorporated solid and relatively stiff intramedullary fixation stems. These stems are fabricated of suitable, compatible metallic alloys and are generally integral with the prosthetic neck and head portions. Therefore, the stems of these components are stiff, that is, they do not provide significant flexure along the length of the stem. However, the surrounding support of cortical bone within which they are implanted does offer some flexibility. Therefore, stiff stems, relative to the more flexible structure of the cortical bone, result in a composite structure wherein the flexural rigidity of the constituent parts varies significantly, depending upon prosthesis stem factors related to sectional size, shape, thickness and material modulus of elasticity.
The use of relatively stiff intramedullary stems has been clinically suspect of producing adverse and destructive bone reactions over a long period of time. More particularly, stiff stems can be attributed to producing micromotion at the stem and bone interface and can also be attributed to the development of reduced levels of force or stress shielding within the surrounding support of bone structure. Both the presence of interlace micromotion and reduced bone stresses can result in adverse bone reactions which have been attributed to the diminution of bone mass at the interface and also within the surrounding bone matrix. Understandably, loss of bone is detrimental to the function of the implant and can produce loosening of the prosthesis and accompanying loss of articular joint or hip function and also results in severe pain. Depending upon the severity of these functional factors, surgical revision may be indicated.
The reaction forces between an implant stem and supportive bone are preferably distributed in such a manner that the greater forces are transferred proximally and decrease uniformly along the length of the fixation stem distally. This force distribution allows a greater proportion of joint reaction force to be transferred to the surrounding supportive cortical bone to levels comparable to that of an intact femur. It is most important for maximum advantages to have a uniformly flexible stem with relatively greater flexibility proximately than distally.
This therefore avoids adverse postoperative bone reaction which has been attributed to the stress shielding phenomenon of relatively stiff conventional hip stems. However, conventional relatively stiff stems reduce the forces distributed to surrounding bone to levels significantly below normal anatomical levels of an intact femur. Therefore, under the influence of reduced levels of bone stress distribution incident to stiff conventional stems, adverse bone reaction may occur postoperatively where the adjacent bone structure degenerates, diminishes or atrophies. This resultant bone loss can seriously affect the structural integrity of the adjacent supportive bone and may ultimately lead to significant loss or compromise of stem fixation. The loss of stem fixation will eventually compromise the long-term function of the implant prosthesis if the resulting pain and/or also loss of function becomes significantly intolerable to the patient.
The present invention therefore incorporates a flexible fixation stem which is integral to the metal prosthetic component neck and head portions. This stem flexibility significantly reduces the micromotion at the stem and bone interface while increasing the levels of force transferred to the adjacent supportive bone structure. This reduction of interface micromotion and the attainment of higher levels of bone force or stress over the entire bone can significantly improve the attainment and sustainment of stem fixation in a number of cementless modes, such as bony ingrowth fixation or press fit fixation. Heretofore the use of relatively stiff stems as are present in prior art and known devices now in use compromised good long-term results in hip joint reconstruction, where the adjacent bone has been adversely affected by the presence of intolerable micromotions at the fixation interface and from stress shielding within the adjacent supportive bone structure.
Accordingly, a number of devices have been utilized which attempt to provide a prosthesis having some flexibility. Examples of these devices may be found in U.S. Pat. Nos. 4,530,114, "Total Hip Joint Prosthesis", issued Jul. 23, 1985 to Tepic; 4,287,617, "Femoral Pin for Hip Prosthesis", issued Sep. 8, 1981 to Tornier; 4,261,063, "Titanium or Titanium Alloy Pin to be Fixed in Long Bones", issued Apr. 14, 1981 to Blanquaert; 3,965,490, "Femural Insert for Hip Joint Prosthesis", issued Jun. 29, 1976 to Murray et al: and 3,893,196, "Body Implant Material", issued Jul. 8, 1975 to Hochman.
The Tepic reference provides for flexure through the use of tension transmitting wires. The Tornier reference utilizes bent sheet metal having a longitudinal slit therein, in an attempt to give the prosthesis some elasticity or flexibility. The Blanquaert reference utilizes a lattice of titanium wire in an attempt to have a modulus of elasticity close to that of the cortical bone tissue. The Hochman reference, while directed to an implant material, attempts to produce an implant material having a module of elasticity which is comparable to that of the cortical bone.
The Tornier reference shows a U-shaped sheet metal femoral component with an opening on the lateral side. The reason for this design is to provide a relatively stiff proximal end. The shape is designed with the objective to provide transverse elasticity, thus having the intent of facilitating positioning of the stem into the medullary canal and thus give a tight fit with the anticipated advantage of a certain "springiness" to the inserted device.
The Hochman et al patent describes a graphite or boron fiber-plastic composite fabrication of several conventional hip prosthesis design, a hip fracture fixation device and an intramedullary fracture fixation rod for long bones. The stem portion of the prosthesis will be somewhat more flexible than its equivalent metal counterpart, but this is accomplished in a different manner by utilizing a more compliant, flexible material of construction.
Other prior art which attempts to disclose and describe devices similar to, related to, or having one or more features of this invention include, U.S. Pat. Nos. 2,066,962, "Shaft for Golf Clubs or the Like", issued Jan. 5, 1937 to Cross; 4,375,810, "Joining Element for Fixation of Bone Tissues", issued Mar. 8, 1983 to Belykh et al; and 4,562,598 "Joint Prosthesis" issued Jan. 2, 1986 to Kranz, German patent no. 2,933,237 to Hoogeveen et al; French patent no. 2,483,218 to Cuilleron; German patent no. 2,636,644 to Heibler et al; German patent no. 2,558,446 to "Pifferi et al; German patent no. 2,015,324 to Timmermans et al; European patent no. 0065481 to Anapliotis et al and European patent no. 0077868 to Godolin.
None of these devices describe or suggest the invention of this application or the devices described and claimed herein.
The Hoogeveen et al patent describes a hollow stem which has a modulus of elasticity conforming to the modulus of the surrounding bone. First of all, in this patent, the term "modulus of elasticity" is somewhat incorrectly used. It represents a property of an elastic material such as metal or bone which is the ratio of stress to strain. By using such a term, the inventor has inferred that the stem of invention device has a stiffness or flexibility of that of the surrounding bone. Thus, the inventors state that in their device the shank component in the proximal section must have particularly high rigidity from its contour and nature of the material. In the bottom section the shank should have only low rigidity, or only a low modulus of elasticity.
On the other hand and quite the opposite in structure, and result, in the device of this invention, the stem at the distal end may be intentionally solid and rigid. Preferably, for maximum cementless fixation advantage, the flexible end is the proximal end with more rigidity in the distal end to minimize interface shear and maximize low transfer to the supporting cortical bone as required to maintain stem support by osseous integration or press fit. If severe osteoporosis is present, one advantage would be to have the entire stem flexible, since all the intact bone is flexible from bone (calcium) loss and flexual matching is especially important for the entire length of the device stem.
The Cuilleron patent shows a femoral prosthesis component which incorporates a sagittal slot. The stated purpose of this slot is to "make the stem elastic, expanding in the medullary canal so as to anchor it without the use of any sealing agent" (bone cement). The forces normally acting on the hip joint in use would in fact, react to close the split stem and therefore would obviously produce a loss of stem fixation from the ensueing collapse of the structure.
The Heibler et al patent describes a composite fiber hip prosthesis of carbon ceramic, MOS.sub.2, or aromatic polyamide fibers combined with a compatible matrix of polyamides or expoxy resin. The hip stem has a hollow distal end to improve intramedullar fixation. Here again, the flexible portion is distally, not proximally used and is incorporated to improve stem fixation within the bone.
The Pifferi et al reference also does not suggest or describe the features of the invention and neither do the Timmermans et al Kranz, or Anapliotis patents relate to this invention except to generally disclose well-known hollow stem devices.
The Godolin European Patent shows a device in which the stem has incorporated therein an expansion screw-actuated taper mechanism to provide a tight fit within the medullary canal of the femur. Frustro conical segments expand radially upon tightening of a screw connected to an expansion cone and mandrel interface. Further, as described, the orientation of the frustro-conical segments are inappropriate to provide stability of the implant or preventing subsidence of and within the bone implantation in situ. If the screw arrangement were reversed, the operability of the device would be more positive.
However, all of these devices have one or more disadvantages in that they are expensive to manufacture, difficult to manufacture, unworkable from a clinical standpoint, or the like. Furthermore, none of them incorporate, describe, or suggest the devices disclosed herein for this invention nor do they give the advantages obtained by use of the herein described devices.